Agriculture Reference
In-Depth Information
ruminants) animals needs to be supple-
mented with inorganic P to meet their
nutritional requirements. Although this is a
rather cheap solution, this practice increases
the amount of P leaching into the environ-
ment (Hamada
et al
., 2005; Gontia
et al
.,
2012).
Besides inorganic P supplementation,
another solution to improve the P content of
feedstuf s is to add the microbial enzyme
phytase to the feed. h e action of this
enzyme on the phytate-protein complex,
one of the primary storage forms of phytate
in seed, releases up to 50% of the P content
of seeds, as well as bound metal cations
(Raboy, 2001). Phytase supplementation
therefore improves the bioavailability of
essential elements for animals and reduces P
pollution through animal manure. However,
the introduction of phytase in feedstuf s
increases feeding costs signii cantly and
imposes some restrictions to diet formu-
lation. Other approaches for improving P
and mineral availability in animal feed
include:
of the phytate biosynthetical pathway. h is
distinction is important, since if both
approaches lead to plants with a low-phytate
phenotype, only those with a high phytase
level can be used as a feed additive to
hydrolyse the phytate contained in other
crops. Two crops, maize and soybean,
account for half of the events under research,
an indication that these transformations are
mostly animal feed-oriented. h e rest of the
events concern wheat, barley, lucerne,
rapeseed and rice.
h e i rst low-phytate GM event to reach
the market is likely to be the GM maize
developed by a Chinese biotech company
(see Table 12.1). h is transgenic maize
transformed by a fungal gene,
phyA2
,
displays a 50-fold increase in phytase
expression, is stable over several generations,
has no impaired germination (Chen
et al
.,
2008) and, most important, is the i rst GM
maize to go through the i ve stages of the
regulatory process in China. h is event has
been introduced in two maize hybrids, and
their commercialization in China is pending
approval from the government. Another
maize expressing an
E. coli
phytase gene
went through various i eld trials in the USA,
and feeding trials with weanling pigs have
shown that it is as ei cient as the
supplementation of feed with phytase to
improve the growth performance of a
P-dei cient diet (Nyannor
et al
., 2007). Other
events are under development in the USA
and in Germany, relying on the introduction
of a
phyA
gene (Drakakaki
et al
., 2005), on
the silencing expression of a transporter
involved in the production of phytate (Shi
et
al
., 2007) or on the use of zinc-i nger
nuclease to disrupt the
IPK1
gene, which
encodes an enzyme that catalyses the
biosynthesis of phytate in maize seeds
(Shukla
et al
., 2009).
Soybean is the second crop that has
received a lot of attention from researchers
willing to reduce its phytate content. One of
the i rst studies has already shown that the
transformation of soybean with a fungal
phytase
phyA
gene is an ef ective approach
to improve P availability of feed, while
reducing P excretion by 50% (Denbow
et al
.,
1998). Recently, the same approach also led
1.
h e activation of endogenous phytase in
grains prior to feed processing.
2.
h e transformation of plants with a
mutant
lpa
gene responsible for a low phytic
acid phenotype.
3.
h e introduction of a transgene for the
production of exogenous phytase in crops.
4.
h e genetic modii cation of livestock for
the production of phytase (Brinch-Pedersen
et al
., 2002; see also Chapter 7).
We focus here on the pipeline for GM plants
with a low-phytate phenotype aimed at
improving the quality of animal feed.
The pipeline for low-phytate crops
h ere are about 20 GM events with low
phytate content currently in the pipeline, at
dif erent development stages. h ey follow
two main approaches. h e large majority of
events come from the transformation of a
target plant with an exogenous phytase
phy
transgene, whose origin can be either
bacterial or fungal, or from yeast, while
some other events are based on the silencing
